time: fix DelayQueue rewriting delay on insert after Poll::Ready (#2285)

When the queue was polled and yielded an index from the wheel, the delay
until the next item was never updated. As a result, when one item was
yielded from `poll_idx` the following insert erronously updated the
delay to the instant of the inserted item.

Fixes: #1700

tokio/src/time/delay_queue.rs

@@ -721,15 +721,16 @@ impl<T> DelayQueue<T> {
                 self.poll = wheel::Poll::new(now);
             }
 
-            self.delay = None;
+            // We poll the wheel to get the next value out before finding the next deadline.
+            let wheel_idx = self.wheel.poll(&mut self.poll, &mut self.slab);
 
-            if let Some(idx) = self.wheel.poll(&mut self.poll, &mut self.slab) {
+            self.delay = self.next_deadline().map(delay_until);
+
+            if let Some(idx) = wheel_idx {
                 return Poll::Ready(Some(Ok(idx)));
             }
 
-            if let Some(deadline) = self.next_deadline() {
+            if self.delay.is_none() {
-                self.delay = Some(delay_until(deadline));
-            } else {
                 return Poll::Ready(None);
             }
         }

tokio/tests/time_delay_queue.rs

@@ -410,6 +410,39 @@ async fn insert_before_first_after_poll() {
     assert_eq!(entry, "two");
 }
 
+#[tokio::test]
+async fn insert_after_ready_poll() {
+    time::pause();
+
+    let mut queue = task::spawn(DelayQueue::new());
+
+    let now = Instant::now();
+
+    queue.insert_at("1", now + ms(100));
+    queue.insert_at("2", now + ms(100));
+    queue.insert_at("3", now + ms(100));
+
+    assert_pending!(poll!(queue));
+
+    delay_for(ms(100)).await;
+
+    assert!(queue.is_woken());
+
+    let mut res = vec![];
+
+    while res.len() < 3 {
+        let entry = assert_ready_ok!(poll!(queue));
+        res.push(entry.into_inner());
+        queue.insert_at("foo", now + ms(500));
+    }
+
+    res.sort();
+
+    assert_eq!("1", res[0]);
+    assert_eq!("2", res[1]);
+    assert_eq!("3", res[2]);
+}
+
 fn ms(n: u64) -> Duration {
     Duration::from_millis(n)
 }